1. Field of the Invention
This invention relates to an optical amplifier for use in an optical transmission system or an optical signal processing system.
2. Description of the Related Art
A laser diode amplifier (LD AMP) or a rare-earth doped optical fiber amplifier have been used as optical amplifiers for optically amplifying an input optical signal to generate a high power optical signal. However, the gain of these amplifiers decreases in proportion to an increase in the optical power of the input signal. Furthermore, when an attempt is made to increase an operating input signal level, the linearity of input to output for the amplifier is disturbed. In order to overcome these problems, a feedback system which detects the input optical signal and output optical signal of an optical amplifier and controls a driving current, in the case of a laser diode amplifier, or controls a pump light source, in the case of a rare-earth doped optical fiber amplifier, has been developed to achieve a constant gain for the optical amplifier.
However, these conventional optical amplifiers have the unavoidable disadvantages of a decreased output level and deteriorated S/N (signal-to-noise) ratio because:
(1) a part of the signal light is lost in the couplers provided at the input and the output sections of the optical amplifier; PA0 (2) the increased number of connections with the couplers; and, PA0 (3) excessive signal light loss in the couplers.
Moreover, the optical system and the circuit for controlling the optical amplifier are complicated. These optical amplifiers are also defective because control is performed based on a comparison between the input optical signal and the output optical signal; and therefore, the optical amplifiers cannot be controlled when there is no optical signal. Because the input optical signals are conveying information, there are times in these intensity modulation systems when the optical signals become zero. Thus, control will not be performed at these times. Additionally, the time constant of the control system must be carefully determined with due consideration to statistical characteristics of the input optical signals. Further, as the output optical signal includes spontaneous emission, which has been amplified, control error inevitably increases in the optical amplifier with larger gains.